1. Field of the Invention
The present invention relates to a radiation image recording and reproducing method, and more particularly, to a radiation image recording and reproducing method employing a divalent europium activated barium fluorohalide phosphor.
2. Description of Prior Art
For obtaining a radiation image, there has been conventionally utilized radiography employing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and an intensifying screen. As a method replacing the above-mentioned conventional radiography, a radiation image recording and reproducing method employing a stimulable phosphor stated, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. The radiation image recording and reproducing method involves steps of causing the stimulable phosphor to absorb a radiation having passed through an object or having radiated from an object; exciting the phosphor with an electromagnetic wave such as visible light or infrared rays (stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose as compared with the conventional radiography. Accordingly, the radiation image recording and reproducing method is of great value, especially when the method is used for medical diagnosis.
As for a stimulable phosphor employable in the radiation image recording and reproducing method, the above-mentioned U.S. Pat. No. 4,239,968 discloses an alkaline earth metal fluorohalide phosphor having the following formula: EQU (Ba.sub.1-x M.sup.II.sub.x)FX:yA
in which M.sup.II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cl, Br and I; A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er; and x and y are numbers satisfying the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2, respectively.
A divalent europium activated barium fluorobromide phosphor (BaFBr:Eu.sup.2+), which is included in the above-mentioned stimulable phosphor, emits light of high luminance (stimulated emission) and the peak wavelength of the emission is approximately 390 nm, and hence the phosphor is practically very useful. It is also known that the stimulation spectrum of the phosphor has the maximum intensity at approx. 600 nm.
The radiation image recording and reproducing method employing a stimulable phosphor, as described hereinbefore, is advantageous for obtaining a radiation image as a visible image. However, it is desired that the sensitivity of the method to a radiation is further enhanced to decrease the exposure dose for a human body or facilitate the following electrical processing. Especially when the radiation is applied to a human body, the enhancement in the sensitivity is of much value from the viewpoint of adverse effect of the radiation on the human body, even if the level of enhancement is not so remarkable. Accordingly, the stimulable phosphor employed in the radiation image recording and reproducing method is desired to give a stimulated emission of luminance as high as possible.